This is safer than just dropping the backend, as .close() will
block if something else is holding the mutex. Also means we can
drop the extra I18nBackend code.
Media syncing still needs fixing.
We no longer need to worry about pysqlite implicitly beginning
transactions, and can be more explicit about beginning/ending
transactions
save() now also has a trx argument controlling whether a
transaction should be started / left open
committing the Protobuf implementation for posterity, but will replace
it with json, as Protobuf measures about 6x slower for some workloads
like 'select * from notes'
- Adding files inside Anki now updates the media DB, so a full file
scan at sync time is no longer required if no other changes have been
made.
- Use a protobuf message for backend initialization, and return a
string error if initialization fails.
